The effectiveness of nature-based therapy for community psychological distress and well-being during COVID-19: a multi-site trial

During the COVID-19 pandemic, the world population faced various mental health challenges, highlighting a need for new community-based psychosocial interventions. This study aimed to investigate the effectiveness and feasibility of Nature-Based Therapy (NBT) for the community experiencing psychological distress during the pandemic. A multi-site trial comparing NBT and control groups was conducted in Korea with 291 participants exhibiting mild to severe depression or anxiety. A total of 192 participated in 30 sessions of therapeutic gardening, while 99 remained in the control group. Psychological distress and well-being were assessed using seven measures of depression, anxiety, daily activity, life satisfaction, mindfulness, stress, and loneliness. The effect sizes (Cohen’s d) of NBT compared to the control group were medium to large: depression (0.583), anxiety (0.728), daily activity (1.002), life satisfaction (0.786), mindfulness (0.645), stress (0.903), and loneliness (0.695). Multilevel analysis revealed significant Time × Group interaction effects for all measures. Pearson correlation (r = − 0.28 to 0.71) showed that changes in all variables correlated significantly with each other, with small to large effect sizes. Therapeutic alliance at post-test positively moderated the intervention effects on the outcomes. We concluded that NBT is a promising psychosocial intervention for treating psychological distress for community dwellers.

www.nature.com/scientificreports/In psychosocial intervention, the therapeutic alliance is considered an important factor for successful treatment.A therapeutic alliance generally refers to the positive and collaborative relationship between a therapist and client (or patient).Therapeutic alliances are regarded as an important aspect of both individual and group psychological treatments 56,57 .However, there is little known about therapeutic alliances in the context of NBT.In studies on wilderness therapy, the relationship between therapeutic alliances and outcomes has been controversial.One study suggested that therapeutic relationships were key change agents for participants 58 , but other studies reported non-significant effects of therapeutic alliances on treatment outcomes 59,60 .The Buddhist psychotherapy perspective emphasizes the therapeutic alliance in NBT, considering as a triangular relationship (therapist, client, nature), therefore, the client perceives and interacts with the natural environment as a real presence, not a psychological phenomenon 61 .Despite the importance of therapeutic alliances in NBT, the empirical evidence is insufficient.Nonetheless, the therapists' ability to guide clients to engage in nature-based activities and build therapeutic alliances is considered important for better outcomes in NBT 62,63 .
Therefore, the present study has three objectives.First, we investigated the effects of nature-based therapy on the psychological distress and well-being of general public during the COVID-19 pandemic using a moderate sample and validated measures.We hypothesized that the psychological distress and well-being of participants in the therapeutic gardening program would significantly improve compared with those of the control group.Depression, anxiety, stress, and loneliness, which were prevalent negative emotional responses during the Covid-19 period 64 , were selected as variables for measuring psychological distress.Additionally, drawing on previous research we incorporated well-being variables encompassing vitality and life satisfaction.Moreover, considering that mindfulness components are often integrated into nature and are closely related to depression and anxiety, we included mindfulness as well-being measures.Second, we investigated the association between changes (post-score minus pre-score) in psychological distress and well-being.We hypothesized that changes in each psychological distress and well-being variable would associate significantly with each other.Finally, we examined the impact of therapeutic alliances on NBT.We hypothesized that the greater the level of treatment alliance, the greater the improvement in psychological distress and well-being.By examining these hypotheses, this study aimed to investigate how effective and feasible it is to provide NBT in the community during the COVID-19 pandemic for the people experiencing mental health challenges.

Sample characteristics
Participants' characteristics, including basic demographics, employment, marital status, mental disorder diagnosis, and education level, are presented in Table 1.Most participants were female (n = 225, 77.3%) and nearly half were married (n = 128, 44.0%).The mean age was 53.48 (SD = 24.05),with no statistical difference between the gardening (M = 52.21,SD = 24.04)and control (M = 56.04,SD = 23.99)groups.Among a total of the 291 participants, 192 were assigned to the gardening group and 99 to the control group.The Wilcoxon test was used to test the significance age difference between the two groups, since it did not satisfy the assumption of normality, and chi-square tests were conducted for other demographic features.The two groups did not statistically differ in gender, age, education, and mental disorder diagnosis, except for marital status [χ2 (4) = 9.973, p = 0.041].There were no significant differences in the baseline mean scores for any of the psychological distress and well-being variables as a result of the independent t-tests (all p > 0.05).

Effects of gardening on psychological distress and well-being
The results of the multilevel analysis are presented in Supplementary Table S1, and the mean scores at each time point, effect sizes (Cohen's d), and statistical significance (p-value) of the ANCOVA are presented in Supplementary Table S2.Significant interaction effects were observed in the time × group (gardening vs. control) for all psychological distress and well-being measures (MHS:D, MHS:A, CORE, SWLS, MAAS, PSS, ULS-8) (all p < 0.01; Table S1).Specifically, the two groups did not statistically differ at baseline; however, the gardening group showed significant improvements in all psychological distress and well-being variables after the intervention, whereas the control group did not (Fig. 1).Effect sizes (Cohen's d) were medium for the MHS:D (d = 0.583), MHS:A (d = 0.728), SWLS (d = 0.786), MAAS (d = 0.645), and ULS-8 (d = 0.695), and large for the CORE (d = 1.002) and PSS (d = 0.903).Even after controlling for the effects of marital status as a covariate, gardening effects were maintained for all psychological distress and well-being variables (Table S2).Although there were no significant differences between the groups in other demographic features, ANCOVA was further conducted after adjusting for age, sex, employment, education, and mental disorder diagnosis, considering the possibility of other potential confounders.And the results showed that there were still significant differences between two groups after the intervention even after adjusting demographic variables (all p < 0.05).

Relationships between psychological distress and well-being variables
All difference scores (post-test minus pre-test) of the psychological distress and well-being variables correlated significantly with each other, with moderate-to-large effect sizes (all p < 0.01, Table 2).Specifically, depression, anxiety, and stress showed large correlation coefficients with statistical significance: depression-anxiety (r = 0.70), depression-stress (r = 0.66), and anxiety-stress (r = 0.70).Daily activity, life satisfaction, and mindfulness also had large positive correlations: daily activity-life satisfaction (r = 0.71), daily activity-mindfulness (r = 0.55), and life satisfaction-mindfulness (r = 0.61).Loneliness was significantly correlated with anxiety (r = 0.50), daily activity (r = − 0.50), and life satisfaction (r = − 0.55), with a large effect size.

Effects of therapeutic alliance on psychological distress and well-being variables
The mean scores of therapeutic alliances for the gardening group at each time point and the results of the Pearson's correlation analysis between therapeutic alliance and psychological distress and well-being variables are presented in Table 2.The mean therapeutic alliance score at T2 (M = 45.16,SD = 12.66) was higher than that at T1 (M = 39.60,SD = 10.14).The Pearson correlation results showed significant associations between the posttreatment (T2) therapeutic alliance scores and the intraindividual pre-post score differences in all the psychological distress and well-being variables (all p < 0.01): depression (r = − 0.38), anxiety (r = − 0.40), daily activity (r = 0.40), life satisfaction (r = 0.52), mindfulness (r = 0.52), stress (r = − 0.51), and loneliness (r = − 0.36).Similar results were observed in the correlation analysis between the average of mid-and post-treatment (AVE) therapeutic alliance scores and psychological distress and well-being variables, while mid-treatment (T1) therapeutic alliance scores did not significantly correlate with any variables.Given that significant associations were found between post-treatment (T2) therapeutic alliance scores and pre-post difference scores for all psychological distress and well-being variables, further multilevel moderation analysis was conducted.Therapeutic alliances (T2) had significant moderating effects on all measures of psychological distress and well-being (all p < 0.01).Table 3 presents the results of the moderation analysis for all variables.

Discussion
This study examined the effects of nature-based therapy on the psychological distress and well-being of individuals with depressive and anxiety symptoms during the COVID-19 pandemic.Overall, NBT showed significant treatment effects on psychological distress and well-being compared with the control group.As hypothesized, all psychological distress and well-being variables of participants in the therapeutic gardening program significantly improved compared with the control group.This result is consistent with many previous studies reporting the positive effects of NBT on psychological distress and well-being [42][43][44]65 . Theeffects sizes found in this study, which are 0.583 for MHS:D, 0.728 for MHS:A, 1.002 for CORE, 0.786 for SWLS, 0.645 for MAAS, 0.695 for ULS-8, and 0.903 for PSS, were similar or larger than the effect sizes (0.35 to 0.95) reported in previous meta-analysis studies of NBT 16,41,43 .
Regarding psychological distress, the same result was derived from previous studies reporting that NBT showed a larger effect size for anxiety than for depression 16,42 .However, since many studies have consistently reported the effectiveness of NBT for depression, it would be appropriate to consider NBT as effective for general mood disorder symptoms rather than concluding that it is more effective for anxiety than for depression.Interestingly, stress was the most improved variable among negative affect variables (depression, anxiety, and loneliness).Considering that previous studies have continuously reported the effectiveness of NBT in reducing stress, stress seems to be easily alleviated by NBT.In addition to self-reported stress levels, several studies have investigated the physiological effects of NBT in reducing stress using EEG, blood pressure, pulse rate, and the immune system 66,67 .Moreover, NBT was found to be effective in improving the quality of life and reducing burnout in stress-related mental illnesses 65 .Although the mechanism of NBT in relieving stress should be further studied, from the Stress Reduction Theory's view, its effects can be explained by the fact that restorative responses to non-threatening nature have benefited human beings during evolution, including the rapid attenuation of stress responses 45 .
Positive mental health and well-being aspects such as daily activity, life satisfaction, and mindfulness improved significantly, as did negative affect.In line with previous studies, NBT had a positive impact on life satisfaction.Interestingly, despite the absence of mindfulness-focused practices within the therapeutic gardening program, participants' levels of mindfulness improved.This could be explained by the possibility that similar mental process to mindfulness occur during nature-based activities, or it may be because the attention to the nature stimuli reduces mind wandering 68,69 .In addition to the several studies suggesting that NBT is beneficial for increasing vigor or physical activity, daily activity level (vitality) showed the greatest improvement in this study among all psychological distress and well-being variables.We used the Core Life Activities Index (CORE) for vitality, which assesses five aspects of daily activity: sleep, eating, physical activity, spending time with friends and family, and new learning, rather than physical activity or vigor.NBT can be considered more effective in enhancing vitality indicating the energy in daily life aspect, rather than to the vitality defined in terms of physical activity levels observed in other studies 70,71 .Loneliness, which represents social aspects of well-being, significantly decreased, showing results similar to those of other studies on the psychosocial effects of NBT 72,73 .In the case of groupbased NBT, gardens serve as vital social arenas, thus appearing to offer additional psychosocial benefits, including reduced loneliness and improved social bonds, compared to home gardening or individual horticultural therapy 73,74 .In the study comparing the effects of individual and group horticultural interventions, group-based intervention showed significantly higher improvements in the socially related sub-scores of the quality of life and emotional intelligence (social score, interpersonal score, and empathy score) 74 .Alternatively, given the significant Table 2. Means, standard deviations, and correlations of therapeutic alliances and difference scores of psychological distress and well-being variables.T1: Therapeutic alliance score at mid-intervention, T2: Therapeutic alliance score at post-intervention, AVE: average of T1 and T2.M and SD are used to represent mean and standard deviation of difference scores, respectively.Each mental health and well-being variable means difference score (post-test minus pre-test).*p-value < 0.05, **p-value < 0.01.www.nature.com/scientificreports/moderating effect of the therapeutic alliance on the reduction of loneliness, the social relationship or bonding with the therapist may have influenced the decrease in loneliness.Since our study aimed to examine the effects of the NBT as compared to the control condition, it would be beyond the scope of the current study to conclude whether the benefits of the NBT were associated with socializing components or exposure to nature.Considering the previous studies indicating that social interactions were significantly higher after participation in social and therapeutic horticulture or gardening, it should be investigated whether enhanced social interaction during the NBT serves as a therapeutic mechanism along with other potential contributing factors, such as mindfulness 68,69 , and exposure to nature in a future study.
We also investigated the association between changes (post-score minus pre-score) in seven psychological distress and well-being variables: depression, anxiety, daily activity, life satisfaction, mindfulness, stress, and loneliness.As hypothesized, significant correlations were found between all the variables.This implies that people experience changes in psychological distress and well-being through comprehensive interactions and not independently.Psychological distress variables (depression, anxiety, and stress) showed large correlations with each other, while positive mental health and well-being variables (daily activity, life satisfaction, and mindfulness) correlated with large effect sizes.This is probably because people suffering from psychological distress experience a combination of negative emotions and improvements occur simultaneously through the intervention.www.nature.com/scientificreports/Similarly, there were significant associations among depression, anxiety, and stress in studies on the psychological impact of COVID-19 75,76 .Regarding positive mental health and well-being variables, it is probable that people experience vitality and mindfulness while participating in gardening, which improves their life satisfaction.In a study on the effectiveness of therapeutic gardening as behavioral activation, vitality mediated the improvement of quality of life, depression, and anxiety 77 .The relationship between psychological variables and the mediating effects of nature-based therapy should be further investigated.The current research team is currently conducting follow-up research with a specific focus on investigating the mechanisms involved.Finally, in line with our hypothesis, therapeutic alliances positively affected psychological distress and wellbeing.Specifically, therapeutic alliance scores at T2 (post-test) were higher than at T1 (mid-test) and had a greater impact on the intervention.This finding is inconsistent with the literature suggesting that early alliance scores are predictive of treatment outcomes 78 .One potential explanation is that the improvement in psychological distress and well-being as the intervention progressed may have enhanced the therapeutic alliance.A therapeutic alliance predicts successive changes in symptoms, and prior symptom changes also affect the therapeutic alliance 79 .This suggests that the therapeutic alliances and treatment effects mutually affect each other.Although alliances in group therapy have a smaller effect than in individual therapy, owing to the dynamics of the group 56 , group cohesion is still an important factor in the outcome of nature-based group therapy 80 .Therefore, therapists must create supportive and connected group environments to achieve better outcomes.
This study had several limitations.First, regarding the experimental design, we did not adopt a randomized controlled trial, and the discrepancy in the number of participants between groups necessitates caution in interpreting the representativeness of the sample and the generalizability of the study results.Since the research was conducted during the pandemic period, it was difficult to recruit participants and randomize them.While COVID-19 had subsided, vulnerable populations, such as those with mental health problems or the older adults, still had a fear of face-to-face contact with people and hesitant to return to social life 81 .Since people were engaged in limited face-to-face activities at their institutions, there were significant concerns about randomizing them to nature-based interventions or treatment as usual.Thus, it was deemed unethical to randomize them.To cope with this limitation, we recruited participants (N) that could derive sufficient statistical power, and attempted to control for individual and group variables using a multilevel model.To provide robust evidence for NBT, studies should have larger sample sizes to avoid type II errors, accurately discover differences between groups, and use reliable measures to ensure strong internal validity 15 .Therefore, we tried to recruit as many participants (N) and employed validated measures.Also, non-randomized controlled designs could provide adequate evidence as alternatives to RCT, and is more convincing when confounders are well-understood, measured and controlled, there is evidence for causality between intervention and outcomes, and effect sizes are large 82 .In this regard, we selected outcome variables by investigating various existing literatures.We have tested differences in demographic data and psychological distress and well-being variables at baseline between the two groups and found no differences, except for marital status.All statistical analyses were conducted after controlling for marital status, and significant large effect sizes were obtained.Nevertheless, there still is a possibility that the motivation of some gardening group participants affected the treatment effect.Second, we were unable to conduct follow-up assessments because of limitations in the study duration.Conflicting results have been reported regarding the long-term effects of nature-based treatments 13,43 , and it is necessary to conduct a follow-up assessment in future studies.Finally, since the intervention was conducted at a time when social restrictions caused by the COVID-19 were gradually eased, there may have been more psychosocial effects.Therefore, it is needed to be cautious in interpreting the effectiveness of NBT during this period.
Despite these limitations, the results of this study contribute to the effectiveness of nature-based therapy, represented by a therapeutic gardening program, as a psychosocial intervention for community dwellers suffering from psychological distress during COVID-19.There are very few experimental studies in the field of NBT research with a sample size exceeding 200 14,16,43 , and during the COVID-19 pandemic, most studies investigating the impact of nature-based activities on mental health and well-being were predominantly survey-based 83,84 .In this regard, the current study resulted in the applicability and feasibility of NBT to local communities by conducting experimental research involving a moderate sample recruited from multi-sites.Additionally, from a methodological standpoint, we tried to control for confound variables, such as site and demographic characteristics, that could potentially influence on the treatment outcomes, with the aim of rigorously examining the effectiveness of NBT.
We concluded that nature-based therapy is an effective and feasible psychosocial intervention and applicable for improving community's psychological distress and well-being.In addition to the applicability of NBT, the current study explored the significant interactive association between changes in psychological distress and wellbeing, which can provide implication for future mechanism studies.Moreover, a later therapeutic alliance was found to play an important role in the intervention outcomes of NBT, similar to other psychosocial treatments.Therefore, future studies on nature-based therapy should focus on the effect mechanism, research methodology of random assignment, therapist competency, and adherence to evidence-based treatments.

Method Study design
We employed a multi-site experimental design with repeated measures to examine the effects of the therapeutic gardening program on the participants' psychological distress and well-being.Depending on the recruitment center, participants were assigned to the gardening or control group.The gardening group participated in naturebased therapy for 30 sessions, whereas the control group did not receive any nature-based activities or intervention.The same outcome measures were employed for both groups except for therapeutic alliances with therapists.
Vol:.( 1234567890 85 and both were utilized in this study for the convenience of the participants.

Mental health screening tool for anxiety disorders (MHS:A)
Severities of anxiety symptoms were measured with Mental Health Screening Tool for Anxiety Disorders (MHS:A) 86 , developed for the timely screening of generalized anxiety disorder (GAD) with relatively high accuracy in the primary medical setting.MHS:A self-report measure that assesses how often symptoms related to generalized anxiety disorder have been experienced in the past two weeks.MHS:A has 11 items scored on a five-point Likert scale (0 = never, 4 = always).Higher total scores indicated a higher level of anxiety symptoms, interpreted as minimal range (0-10), mild (10-20), moderate (20-30), or severe (> 30) anxiety symptoms.MHS:A has been reported to have excellent internal consistency in both online and offline versions (Cronbach's α, offline version: 0.957, online version: 0.956) 86 and both versions were used in this study at the convenience of the participants.

Core life activities index (CORE)
The Core Life Activities Index (CORE), developed by the current research team, was administered to assess the level of vitality in daily activities over the past week.This measure includes five questions about engaging in everyday activities such as sleep, eating, and physical activity, scored on a five-point Likert scale (1 = never, 5 = always).Higher scores indicated greater engagement in daily activities, which can be interpreted as higher vitality.CORE has previously shown good internal consistency in a preliminary study on the psychological effects of COVID-19 in Korea (Cronbach's α = 0.77) 75 .

Satisfaction with life scale (SWLS)
Subjective life satisfaction was assessed using the Korean version of the Satisfaction with Life Scale (K-SWLS) 88 , which was validated using the Korean version of the original Satisfaction with Life Scale (SWLS) 89 .The SWLS is a 5-item measure with a seven-point Likert scale (1 = strongly disagree to 7 = strongly agree).Higher scores indicated greater satisfaction with one's personal life.K-SWLS has shown good internal consistency in the validation study (Cronbach's α = 0.85-0.90) 88.

Mindful attention awareness scale (MAAS)
Mindfulness was assessed using the Korean Version of Mindful Attention Awareness Scale (K-MAAS) 90 , which is a validated version of the original Mindful Attention Awareness Scale (MAAS) 91 .The MAAS is a 15-item measure that assesses mindful attention and awareness in one's daily life using a 6-point Likert scale (1 to 6), with higher scores indicating greater mindfulness.The K-MAAS reported significant internal consistency in a validation study (Cronbach's α = 0.87) 90 .

Perceived stress scale (PSS)
The level of perceived stress in daily life was assessed using the Korean Version of the Perceived Stress Scale 92 , which is a validated version of the original scale 93 .The PSS consists of 10 items that measure the degree to which individuals have perceived and interpreted subjective stress over the past month, using a five-item Likert scale (0 to 4).A higher total score indicated a greater degree of stress experienced by the individual.The Korean version of the PSS showed good internal consistency in a validation study (Cronbach's α = 0.82) 92 .
A short form of the UCLA loneliness scale (ULS-8) Loneliness was measured using a short form of the UCLA Loneliness Scale (ULS-8) 94 , the abbreviated version of the 20-item UCLA Loneliness Scale 95 , which is the most commonly used measure of loneliness.This 8-item measure assesses the subjective sense of being separated from others and is scored on a four-point Likert scale (0 to 3).A higher total score indicated a greater degree of loneliness.The ULS-8 reported good internal consistency in a validation study (Cronbach's α = 0.84) 94 .

Working alliance inventory-short revised (WAI-SR-K)
The Korean version of the Working Alliance Inventory-Short Revised (WAI-SR-K) 96 is the Korean version of the Working Alliance Scale-Short Revised version 97 .The WAI-SR is a 12-item scale that assesses therapeutic alliances.It consists of three subscales with four items respectively: agreement on the goals of therapy, agreement on the tasks of therapy, and the development of an affective bond between therapists and clients.A higher total score indicated a greater degree of therapeutic alliance with the therapist experienced by the client.The WAI-SR-K showed excellent internal consistency in the validation study (Cronbach's α = 0.93) 96 .

Statistical analysis
An independent team performed all statistical analyses.Independent t-tests and chi-square tests were conducted for continuous and categorical variables, respectively, to examine baseline differences in demographic characteristics and psychological distress and well-being variables between the two groups (gardening group vs. control group).Among the demographic features, marital status was found to be non-equivalent between the groups; therefore, ANCOVA was conducted to re-analyze the treatment effects, considering marital status as a covariate.If the data did not satisfy the assumptions such as normality and homogeneity of variance, it was analyzed using a nonparametric method.Since our data were nested as they were collected from 11 sites, a multilevel analysis was used to compare the changes between the two groups.Multilevel analysis, also known as the linear mixed-effects model, allows random intercepts for individuals and sites for differential clustering.Therefore, www.nature.com/scientificreports/individuals (participants) and sites were included as a random effect in our multilevel model.In this model, time (three time points including pre-, mid-, post-test) was included as a within-subject (Level 1) parameter, and individuals (Level 2) and sites (Level 3) were included as between-subject parameters.For effect comparison, the group (gardening vs. control) was set as a moderating variable to examine the interaction effect of the time × group.Group-mean centering was adapted to reduce the risk of multicollinearity and increase the ease of interpretation 98 .Effect sizes (Cohen's d) were calculated for each of the seven psychological distress and wellbeing measures (MHS:D, MHS:A, CORE, SWLS, MAAS, PSS, and ULS-8).Cohen's d can be interpreted as small (d = 0.2), medium (d = 0.5), or large (d = 0.8) effect sizes, based on Cohen's suggestion 99 .Bivariate Pearson correlations between the difference scores (post-score minus pre-score) of each psychological distress and well-being variable and therapeutic alliance were computed to examine the association between changes in each variable and between the therapeutic alliance and changes in psychological distress and well-being variables.According to Cohen's suggestion, the correlation coefficient indicates a small (r = 0.1), moderate (r = 0.3), or large (r = 0.5) strength of the association between two variables.The p-values were adjusted using holm method for multiple correlation analysis to avoid Type I error.After the correlation analysis, a moderation analysis was conducted to examine the impact of the therapeutic alliance on the intervention effect for variables that were significantly correlated with the therapeutic alliance.Statistical analyses were conducted using R software (version 4.2.2).R software was utilized to conduct multilevel analysis, correlation analysis, and calculate effect size using the "nlme" 100 and "effsize" 101 packages.Also, "ggplot2" 102 package was utilized to visualize the results to facilitate interpretation. https://doi.org/10.1038/s41598-023-49702-0

Table 1 .
Baseline characteristics of participants.t = t-value, χ2 = chi-square value, z = z value from twosamples Wilcoxon test.*p-value < 0.05.M and SD are used to represent mean and standard deviation, respectively.MHS:D mental health screening tool for depressive disorders, MHS a mental health screening tool for anxiety disorders, CORE core life activities index, SWLS satisfaction with life scale, MAAS mindful attention awareness scale, PSS perceived stress scale, ULS-8 a short form of the UCLA loneliness scale.

Table 3 .
The result of moderation analysis of therapeutic alliance (T2).Post-intervention.SE = Standard Error.df = degree of freedom.t = t-value.*p-value < 0.01.MHS:D mental health screening tool for depressive disorders, MHS:A mental health screening tool for anxiety disorders, CORE core life activities index, SWLS satisfaction with life scale, MAAS mindful attention awareness scale, PSS perceived stress scale, ULS-8 a short form of the UCLA loneliness scale.